Approximately 1.6 billion tons of steel is produced annually, out of which 1 billion tons is hot iron metal produced from iron ore in the blast furnace production route. The hot iron metal production accounts for 8% of the world's CO2 emissions. Any contribution that will reduce the CO2 emissions will therefore have an enormous impact on the environment.
The major part of all iron produced from iron ore is produced in the blast furnace process. That process has been gradually developed and refined over a time span of at least 300 years and has now reached very close to its theoretical optimum. In spite of this, the process suffers from a number of serious drawbacks. As mentioned above, the CO2 emission is high and the energy efficiency is low, only about 50%. The blast furnace iron production chain also represents an enormous investment with its pelletizing plant, coking plant and blast furnaces with hot stoves and oxygen plants. Therefore, there have been a great number of attempts over the past 75 years to develop alternative processes to overcome these drawbacks and replace the blast furnace. The search for a new more energy-efficient and/or cheaper process has been along one of three routes. The first is solid state reduction processes, such as the “Direct Reduced Iron” process. The second is a combination of pre-reduction in solid state followed by a final smelting process. The third one is a further increase of the energy utilization in the blast furnace process by re-circulation of the partly utilized off-gas after CO2 stripping.
The development along these three routes all utilize the counter-current principles to obtain as high utilization of the energy as possible, but are limited by the equilibriums between the different iron oxides and the reduction gas. All these processes use coal both as reducing agent and energy supply. This means that none of these new iron making processes represent any major breakthrough when it comes to CO2 emissions.
Today most of the alternative processes to the blast furnace process are based on a process that is based on combinations of pre-reduction in solid state followed by a final smelting step. In this type of processes the gas leaving the smelting step is utilized in the pre-reduction step.
Only one of these processes, the Corex process, has reached a commercial operation. An overview can be found in e.g. “Modern processes for the coke-less production of iron” by A. B. Usachev et al., in Metallurgist, Vol. 46, Nos. 03-04, 2002, pp. 117-130. The Corex process consists of two reduction stages. In the first stage, the iron oxide is partly reduced and then followed by a smelting reduction stage to produce a molten metal similar in composition to a blast furnace pig iron. Corex's disadvantage is the need for agglomerated ore, and since coal is used both as fuel and as reducing agent, the volume of off-gases is high, making the downstream off-gas system expensive. This increases not only operating and investment costs but also the CO2 footprint.
Finex, CCF, DIOS and Hismelt are processes that use similar approach as COREX, that is pre-reduction in solid phase and final reduction in a liquid phase. The major difference between these processes and Corex is the type of raw material used. While COREX uses sinter, pellets or lump, these four other processes use fine ore as raw material, which after pre-reduction is transferred to a second, smelting reduction stage producing a liquid metal. Finex and CCF are in experimental phase while DIOS and Hismelt are in demonstration plant phase. All of these processes including Corex operate at high pressures in the liquid-phase reduction step and need to be hermetically sealed. Since the coal is used both as fuel and as a reducing agent, there are similar disadvantages as for the Corex process.
Another attempt for a more substantial decrease in CO2, emission is described in Swedish patent SE 453304 where the preheating, pre-reduction and melting reduction is carried out by partial combustion of the off-gases from the final smelting reduction, all these reactions taking place in the same reactor. The weakness of this process is the unavoidable inter mixing between both gases and liquids from the different zones in the reactor which makes it impossible to balance the generation of CO-gas in the final smelting reduction stage with the need for CO-gas in the other stages. Furthermore the partial combustion of the CO-gas in the preheating—pre-reduction stage makes it unsuitable to inject the gas into the smelting reduction stage as plasma gas.
Even though efforts have been made making the reduction of oxides of iron more energy efficient, e.g. by pre-reduction of the incoming oxide raw material, the amount of CO2 released from the process is still very high. A general problem with prior art methods for reducing iron is that they are inefficient in using the supplied energy and produce large amounts of carbon dioxide emissions, which make the processes environmentally unfriendly and expensive.